Structural material of compressed resin impregnated paper sheets



Patented Sept. 20, 1949 auauz srau C'I'UBAL MATERIAL F comzssnn RESIN IMPBEGNATED PAPER SHEETS Gardner 1!. chidester, Parker R. Baird, George E. Mackin, Forrest A. Simmonds, Clarence 0. Seborg, Mark W. Bray, and John N. McGovern, Madison; dedieatedtothefreenseoftho People in the territory of the United States No Drawing. Application October 2, 1843,

Serial No. 504,780

l Claims. (CL 154-45.!)

(Granted under the act of March 3,

amended April 30, 1928; 370 0. G. 757) This application is made under the act of March 3, 1883, as amended by the act of April 30, 1928, and the and claimed, if patented, may be manufactured and used by or for the Government of the United states of America for governmental purposes without the payment to us of any royalty thereon.

We hereby dedicate the invention herein described to the free use of the People in the territory of the United states to take effect on the granting of a patent to us.

This invention relates to structural materials and more particularly to such materials employing paper as a base, and a method of manufacturing them to impart high mechanical properties,

Laminated paper plastics have been made heretofore, but it has not been possible to manufacture them with sufficient strength to permit their use as structural parts of airplanes, automobiles, buildings, and so forth, that are subjected to high structural stresses. Paper laminated plastics of the prior art are limited in use generally to the taking of vary light stresses and for decorative purposes.

We have found that by following a very careful procedure, paper laminated plastics can be made having enormous strength, being comparable on a weight basis to that of iron, aluminum, steel, and other structural materials, and in many respects more desirable.

In general, the process of our invention em-' braces cooking wood chips from species, such as black spruce, balsam fir or other suitable species yielding pulps making strong papers, to separate the fibers under such mild conditions that the original or native strength of the fibers is substantially preserved and not appreciably diminished. The sulfitejsulfate, or neutral sulfite process as ordinarily used to make chemical pulp in the pulp-making industry may be employed. When using the sulfite process, a relatively low temperature in the range of 120 C. to 135 C. and arelatively long period of time, namely,

. to 20 hours, are used.

Whenthe sulfate process is used, the total concentration of chemicals is relatively low (about 30 grams per liter) and the time of digestion is extended sufflciently toreduce completely the chips to, pulp. However, lower or higher digestion temperatures and shorter cooking schedules may be used by adjusting the concentration of chemicals in the cooking liquor to about 50 to '70 grams per liter.

After cooking, the pulp, if beaten, is not proginvention herein described essed more than to produce a degree of hydration suflicient to permit bonding of the fibers so that the fibers retain as much of their original or native strength as possible. Ordinarily, a hydration index of 750 to 850 cc. Schopper- Riegler freeness is suilicient. It is important to carefully observe this step in the process because the maximum strength cannot otherwise be developed in the plastic product. The fibers thus hydrated are then felted and dried to produce a sheet having high porosity, high density, and high tensile strength.

In producing the sheet, the specially prepared hydrated fibers are fed to the flow box of a paper machine at about 0.2-percent consistence and are flowed onto the Fourdrinier wire at a speed different from that of the wire, which results in alignment of the fibers in the direction of the flow thus producing a highly grained sheet. The difference in speed should be such as to obtain an anisotropy of grain which will provide a ratio of at least 2.0 to 1 in tensile strength. High sheet densities between 0.6 to 0.8 are obtained without materially reducing the porosity of the sheet by using very high pressures on the wet press (from to pounds per lineal inch of nip). Still further densification of the web may be accomplished by adjusting pressure on the calendars. The latter increases the strength but decreases the porosity of the sheet to a marked degree and, therefore, the use of high calendering pressure is limited to sheets of high porosity in the uncalendered condition.

The sheet should be formed very thin, generally less than 4 mils, in order to avoid the orientation of fibers in the third dimension (thickness). Ordinarily, a sheet of paper with a high degree of porosity has a correspondingly low tensile strength, and vice versa. The usual paper-making procedure for developing high porosity in paper is diametrically opposed to that for developing high tensile strength, generally achieved by hydrating the fibers, so that ordinarily a paper possessing one of these qualities to a high degree will possess the other quality to a very low degree. By following the process of this invention, a paper sheet is produced which has both high tensile strength and relatively high porosity. With such a paper, it is possible to fabricate laminated plastics with high structural strength under very low laminating pressures. In other words, the paper itself must be very strong and still be highly porous to permit thorough impregnation of the resin. In making the plastic, the paper is first impregnated with an uncured ther- 3- mosetting or thermoplastic resin, or a mixture of both, and then dried. The dried impregnated paper sheets are then assembled, with either the grain of the paper oriented parallel or alternately crosswise, and the resulting lamination cured under heat and low pressure to form the'laminated paper plastic.

The following examples will illustrate this invention, but it is to be understood that they are not intended to limit in any way the scope thereof.

Example 1 Black spruce wood chips were cooked by the sulfate pulping process-using a liquor of 33.85 percent sulfidity at a total concentration of 30 grams per liter and a total amount of chemicals equivalent to percent of the moisture-free weight of the chips. The temperature of the mixture was raised from 35 C. to 170 C. during a period of 1% hours and heatin was continued at 170 C. for 3 hours. The pulp was then screened and beaten to a Schopper-Riegler freeness index of 835 cc. The beaten pulp was formed into a paper having a density of 0.61 gram per cc., a porosity index of 3.2 seconds, and a thickness of 2.6 mils. 12,850 pounds per square inch in the machine direction and 4,670 pounds per square inch across the machine direction. The paper was then impregnated with an uncured phenol-formaldehyde resin and dried. Sheets of the impregnated paper were assembled in laminations, some. with the grain laid in the parallel direction and others in The paper had a tensile strength of crosswise directions. The laminations were then cured under a pressure of 250 pounds per square inch at a temperature 325 F. The resulting laminated plastics had the following properties when tested in grain for parallel laminates:

F. The resulting laminated plastic was found to have the following in grain properties:

Tensile Same as Example 1, except that the initial concentration of the cookin liquor was increased from 30 to 70 grams per liter and the time of cooking at maximum temperature was decreased from 3v hours to 1.hour. The properties of the final parallel laminated plastic were as follows,

' 20 when tested in grain:

Same as Example 2, except that black spruce chips were used and the cooking was conducted over a period of 3 hours at a temperature of 160 C. The properties of the parallel laminated plastic were as follows, when tested in grain:

Tensile.

strength pounds per square inch 38,790

Example 2 Balsam fir wood chips were cooked by the sulfate pulping process using a liquor having 33.85 percent sulfidity, the total. concentration of chemicals being grams per liter. The temperature of the cooking was raised from 35 C. to 170 C. during a period of 1 hours. The heating was continued at this temperature for 1% hours. The pulp was then screened and beaten to a Schopper-Riegler freeness index of 845 cc. The beaten pulp was formed into a paper having a density of 0.62 gram per 00., a porosity index of 5.2 seconds, and a thickness of 2.6 mils. The paper had a tensile strength of 13,460 pounds per square inch in the machine direction and 4,680 pounds per square inch across the machine direction. The paper was then impregnated with an uncured phenol-formaldehyde resin and dried. Sheets of impregnated'paper were assembled in laminations with the grain oriented in a parallel direction, and cured under a pressure of 250 pounds per square inch at a temperatur of 350 Modulus of elasticity (in tension) do 3,135,000 Modulus of rupture (in bending) d0 30,220 Modulus of elasticity (in bending) do 2,672,000 Density grams per cc 1.33

Example 5 Black spruce chips were cooked by the sulfite pulping process using 57.5 gallons of cooking liquor per pounds of moisture-free wood, the

7 after screening had a Schopper-Riegler freeness index of 860 cc. The pulp without further treatment was formed into a paper having a density of 0.71 gram per cc., a porosity index of 12 seconds, and a thickness of 2.3 mils. The tensile strength oi the paper was 13,860 pounds per square inch in the machine direction and 4,760 pounds per square inch across the machine direction. The

paper was then impregnated with an uncured phenol-formaldehyde resin and dried. Sheets of impregnated paper were assembled in laminations with the grain of paper laid in a parallel direction, and cured under a pressure of 250 pounds per square inch at a temperature of 325 F. The resulting laminated plastic was found to have the following properties, when tested in grain for parallel laminates:

Tensile strength pounds per square inch 38,060 Modulus of elasticity (in tension) do 3,474,000 Modulus of rupture (in bending), do 36,100 Modulus of elasticity (in bending) do 3,293,000 Density grams per 00.. 1.41

Example 6 inch across the machine direction. The physical properties of the final resin-treated, parallel laminated plastic were as follows, when tested in grain: d

"Tensile v strength pounds per square inch 38,050 V Modulus of elasticity ((in tension) do 3,627,000 Modulus of rupture (in bending) do 31,930 Modulus of elasticity.

(in bending) do 3,210,000 Density grams per cc 1.41

Example 8 Black spruce chips were cooked by the sulfite process using the same amount of liquor and conccntration as in Example 6. The chips were presteamed and the temperature of the cooking mixture raised as described in Example 5. The resulting pulp was screened and beaten to a Schopper-Riegler freeness index of 858 cc. The beaten pulp was formed into paper having a density of 0.72 gram per cc., a porosity index of 13 seconds, and a thickness of 2.3 mils. The tensile strength of the paper was 14,480 poundsper square inch in the machine direction and 5,270 pounds per square inch across the machine'direction. The paper was impregnated with uncured phenolformaldehyde resin and dried. Sheets of impregnated paper were assembled in laminations, some with grain laid in parallel and others with grain laid in crosswise directions, and cured as in Example 5. The resulting laminated plastics had the following properties when tested in grain for parallel laminates:

Modulus of electricity (in tension) 338 000 Modulus of rupture (in bending) s. i. s. i Cross laminated.

Parallel laminated. Cross laminated.

35.000 p. s. i 26,935 n. s. i

Modulus creamy an {3:33:83 3; 533 333333?- l.4l grams per cc Parallel laminated.

Density {1.39 grams per cc Cross laminated.

The pulp without further beating treatment was formed into paper having a density of 0.74 gram per cc., a porosity index of seconds, and a thickness of 2.1 mils. The tensile strength in the machine direction was 12,200 pounds per square inch and across the machine direction 4,970 pounds per square inch. The resultin resintreated, parallel laminated paper plastic had the Same as Example 6, except that the pulp was beaten to a Schopper-Riegler freeness index of 850 cc. The paper, made under the special paper making conditions, had a density of 0.74 gram per cc., a porosity index of 22 seconds, and a thickness of 2.3 mils. The tensile strength of the paper was 13,750 pounds per square inch in the machine direction and 4,950 pounds per square Example 9 Balsam fir wood chips were cooked by the sulfite process for making chemical pulp using an amount of liquor equal to 62.5 gallons per pounds ofmoisture-free wood, the liquor having the same "concentration as in Example 5. The chips were steamed and the cooking temperature ranged the same as in Example 5, except that the heating was continued at C. for 4.5 hours. The pulp was blown, screened and beaten to a Schopper-Riegler freeness index of 840 cc. and formed into paper having a density of 0.74 gram per cc., a porosity index of 75 seconds, and a thickness of 2.3 mils. The tensile strength of the paper in the machine direction was 14,350 pounds per square inch and 5,610 pounds per square inch across the machine direction. These sheets were then impregnated with uncured phenol-formaldehyde resin, dried, assembled, and cured the same as in Example 5. The resulting parallel laminated plastic had the following properties, when tested in grain:

Tensile strength pounds per square inch 36,180 Modulus of elasticity (in tension) do 3,114,000 Modulus of rupture (in bending) do 35,330

, Modulus of elasticity a (in bendlgg) do..-.. 3,010,000

Density grams per cc-.. 1.42 Example 10 the wood on a moisture -free basis. The temper.

ature of the cooking mixture was raised from 30 C. to 110 C. in '1 hour and then raised to 181- C. in 4.25 hours at which temperature the heating was continued for 3.75 hours. The pulp was formed into pap r having a density of 0.67 gram per cc., 9. porosity index of seconds, and a thickness of 2.0 mils. The. tensile strength of the paper in the machine .direction was 14,700 pounds per square inchand 5,570 pounds per square inch across the machine direction. The paper was then impregnated with an uncured.

phenol-formaldehyde resin and dried. Sheets of impregnated paper were assembled in both parallel and in crosswise laid laminations and cured at temperature of 325"v F. The resulting plastics had the following properties when tested in grain 30 phenol-aldehyde resin. I

asses 4a modulus: 0r elasticity .0: over about 2,000,000 pounds per square inch are within the scope of this invention.

Having thus described our invention, we claim: v

1. A structural material comprising compressed synthetic resin-bonded laminated sheets of paper; each. sheet being made from pulp having a freeness of 150 to 860 00.: each sheet of paper having a porosity index of about 3.2 to 75 seconds, an anisotropy of grain providing a ratio of at least 2.0 to 1 in tensile strength, and a density of 0.60 to 0.80 gram per cc.', and a thickness of not more than about 4 mils; said sheets having a, distributionof a solid, strengthening synthetic resin in their porous structure which bonds them together; said structural material having a tensile strength of at least 25,000 lbs. per square inch and a modulus of elasticity in tension of over 2,000,000 pounds per square inch density of not less than about 1.33.

2. The structural material definedin claim 1 in 1211011 the synthetic resin is .a thermos'etting res 3. The structural material defined inclaim 1 in which the synthetic resin is a thermosetting phenol-formaldehyde resin.

- a pressure of 250 pounds per square inch at a 4. The structural material defined in claim 1 in which the synthetic resin is a thermosetting 1.38 grams per cc.

for parallel laminates: 5. The structural material defined in claim 1 44 590 s. i Parallel laminated.

Tens strength 271900 3. s. 1-..- 21: ll n nmatet i Modulus o1 elasticity (in tension) 213% B: S: i-

oi... imim aid ul oi ru ture (in bending) 32 600 p. s. i Parallel laminated ul of ela ticity (in bending) 2,975,000 p. s. i- Parallel laminated.

1.37 grams per cm... Parallel laminated.

Density Cross laminated.

Resins. suitable for the production oi. highstrength laminates of P per include: Phenolaldehyde resins (monohydric, dihydric, trihydric, and polyhydric phenols in combination with formaldehyde, benzaldehyde, furfuraldehyde, and other aldehydes, such as are obtained from aliphatic and aromatic acid compounds); aminoaldehydic resins, such as urea-aldehyde compounds, including thiourea and melamine-aldehyde resins; and other thermosetting resins having similar properties.

Paper laminated plastics of great mechanical strength result from the general fabrication process illustrated by the foregoing examples. Heretofore, it was not possible to obtainparallel-laminated paper plastics having tensile strengths of much over 20,000 pounds per square inch and correspondingly small moduli of elasticity, nor

was it possible heretofore to obtain cross-laminated paper plastics having tensile strengths of more than 15,000 pounds per square inch, in grain, and correspondingly small moduli of elasticity. It is to be understood that any paper laminated plastic in which the grain direction of the laminations is parallel, generally referred to as parallei-laminated paper plastics having a tensile strength of over about 35,000 pounds per square inch and a modulus of elasticity in tension of over about 3,000,000 pounds per square inch, is within the scope of this invention. In the case of crosslaminated paper plastic materials, that is, where the grain direction of the paper laminations is alternated, those having a tensile strength of over about 25,000 pounds per square inch and a in which the synthetic resin is a thermosetting urea-aldehyde resin.

6. The structural material defined in claim 1 in which the synthetic resin is melamine-aldehyde resin. 4

GARDNER H. CHHDESTER. PARKER K. BAIRD. GEORGE E. MACKIN. FORREST A. SIMMONDS. CLARENCE O. SEBORG.

MARK W. BRAY.

JOHN N. McGOVERN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,284,432 O'Conor Nov. 12, 1918 1,303,753 .Wright May 13, 1919 FOREIGN PATENTS Number Country Date 284,232 Great Britain Jan. 10, 1929 OTHER REFERENCES and having a a thermosetting 

